Hardboard and method of manufacture



March 26, 1 c. w. HERRINGTON ETAL 3,033,123

HARDBOARD'AND METHOD OF MANUFACTURE Filed May 14, 1958 mm ms e Y EE mm mm CG 2 2 m a 4 m 4 2 3 wwnwm DI! I. b 1 ml 2 m1 mp K Ti m am 5V4. Q w my B United States Patent 3,083,128 HARDBOARD AND METHOD OF MANUFACTURE Charley W. Harrington and George G. Simpson, Laurel, MISS. assignors to Masonite Corporation, Laurel, Miss, a corporation of Delaware Filed May 14, 1958, Ser. No. 735,283 Claims. (Cl. 154-453) The present invention relates to the production of decorative lignocellulose hardboard sheet products characterized by greatly improved resistance to deflection, by increased load bearing ability, and by improved dimensional stability. More specifically, the invention relates to the production of hardboard sheet articles containing integral rib portions which greatly increase the stiflness and stability of the articles without contributing additional weight thereto. At the same time, the rib portions present the pleasing appearance of a plank and batten strip construction. The invention includes both the novel articles themselves and the method of their production.

It has long been recognized by the art that one of the principal deficiencies of hardboard sheet articles resides in their relatively low resistance to deflection under load and their lack of dimensional stability. The lack of stiffness is reflected in the tendency of hardboard sheets to sag and warp in application as shelving, as sofiit board, as roofing, and in like applications. Previous attempts to increase the stiffness of hardboard sheets have been mainly directed to increasing the thickness and/ or density thereof. However, all such approaches have resulted in greatly increasing the weight of the products and the increase in weight has not been accompanied by a corresponding improvement in stiffness. Instability of hardboard sheets under changing atmospheric conditions has been evidenced by warping and buckling of the sheets in normal usage.

It is a. principal object of the invention to provide a novel method for producing hardboard sheet articles characterized by greatly improved resistance to deflection and improved dimensional stability without any substantial increase in the weight of the products.

Another object of the invention resides in the provision of a novel method of imparting to hardboard sheet articles rib portions formed integrally during the sheet consolidation.

A still further object of the invention resides in the provision of hardboard sheet articles presenting a decorative appearance while, at the same time, providing greatly increased stiifness, load bearing ability, dimensional stability, and the like improved physical properties.

Other objects of the invention will become apparent from the following detailed description thereof. The description will be given with particular reference to the accompanying drawing in which FIG. 1 is a perspective view of the novel rib-containing hardboard sheet,

FIG. 2 is a perspective view of a modification of the rib structure,

FIG. 3 is a sectional view illustrating still another rib portion modification, and

FIG. 4 is an enlarged sectional view of a single rib portion.

In carrying out the novel method of the invention, a felted sheet of hydrolyzed lignocellulose is charged to the opening of a hydraulic press equipped with heated platens. The felted sheet may be formed according to conventional methods such as, for example, by following the method described in US. Patent No. 1,663,505 to Mason. In this method, a water slurry of hydrolyzed fibers is flowed onto the traveling screen of a modified Fourdrinier forming machine and thereafter partially dewatered to form a porous felted sheet. The felted fibrous sheet is then consolidated under heat and pressure. The wet, felted sheet may, of course, contain as much as about 50% or more of water by weight when charged to the press opening. In such case, a conventional screen supporting means is employed and the screen openings provide exit for the water. In the method of the present invention, the felted fibrous sheet is consolidated with one surface, preferably its top surface, in contact with an imperforate pressing plate having a plurality of spaced apart, parallel depressions extending thereacross which, when pressed against the sheet, will form a consolidated article having elevated rib portions. The thus formed consolidated sheet, as illustrated by FIGS. 1 and 2, contains a corresponding number of elevated rib portions 10 interconnected by flat depressed areas 12.

Where desired, the above described wet, felted fibrous sheet may first be dried and then consolidated to produce a hardboard article characterized by two smooth surfaces. A suitable method for accomplishing this is described in U.S. Patent No. 2,120,137 to Mason. In employing a relatively dry fibrous lap, inasmuch as no provision need be made for support of the sheet while charging to the press or escape of water therefrom, one surface of the lap may be in contact with a flat imperforate pressing plate while the above-described depression-containing pressing plate contacts the other sheet surface during the pressing procedure. Accordingly, the consolidated product, while still containing the novel integral rib portions, will present two smooth surfaces.

It has also been discovered that great improvement in stiffness, load bearing, stability, and other physical properties is obtained by impregnating the consolidated sheets with a drying oil composition and baking the impregnated sheets to cause polymerization of the oil. The so-termed drying oil may comprise any siccative material including naturally occurring vegetable drying oils as well as. products obtained synthetically which have similar properties.

As would be expected, the rib portions 10 are of lower density than are the depressed areas 12 of the consolidated sheet inasmuch as they are not so highly compressed during the pressing treatment. However, contrary to expectations these areas of relatively low density contribute greatly to the improvement of certain of the physical properties of the hardboard including stiffness and dimensional stability. The reason attributed to this unexpected improvement resides in the fact that the rib portions 10 comprise essentially a plurality of enlarged molded areas. As illustrated in FIG. 4, each of the rib portions 10 consists of a relatively dense outer, or shell, portion 14 surrounding a low density inner, or core, portion 16. While it would be assumcd that the rib portion 10 would be of lower density than the interconnecting areas 12., it would also be presumed that the ribs would be of uniform density throughout their entire thickness. The formation of rib portions, molded in situ during consolidation of the sheet, was entirely unpredictable and completely unexpected. This feature comprises one of the outstanding attributes of the invention. Furthermore, due to the lower density and greater porosity of the rib portions, more drying oil is absorbed thereby than is taken up by the more dense depressed areas. Consequently, the polymerization of the drying oil during the baking treatment lends to the outer shell portion 14 a physical characteristic which may best be described as case hardening. This property is evidenced by the greater increase in density to these shell areas 14 than is obtained in the depressed areas 12 of the novel sheet products as Well as in more pronounced increase in sheet stiflness. In practice, a flat sheet containing no rib portions will normally absorb about 2.5- 6.0% by weight of oil. A similar sheet of the novel rib-containing hardboard will absorb about 3.38.0% by weight of the oil, the increase in uptake being primarily in the area of the lower density rib portion 10. The increased oil uptake of the rib-containing sheet represents an increase of about 25-40% in comparison with a conventional flat sheet article and, as above stated, results in a case hardening effect to the rib portions together with the attendant improvement in physical properties.

Tests have shown, subsequent to oil impregnation and baking of the sheets, that the areas 12 are characterized by a specific gravity density of about 1.08-1.15. The shell portions 14 of the ribs have specific gravities or densities of about 0.77-0.93 and the core portions 16 have specific gravities or densities of about 0.60-0.72.

Prior to impregnation the comparable specific gravities or V densities are: flat areas-1.02 to about 1.07; rib portions (including both shell and core)-O.60-0.68. The rib cores are not appreciably afiected by the tempering treatment. l

Further tests have shown that the load bearing properties of the novel hardboard panels are nearly doubled by the presence of the rib portions. These physical properties are illustrated by the following table. All samples tested were impregnated and baked and then subjected to. standard deflection tests. The samples (6 inches wide) were mounted on a 16-inch span and the load was applied at a uniform rate at the center of the span.

All of the samples containing ribs were mounted with the ribs extending lengthwise of the samples, i.e. transverse to the direction of load application. Stitfness is derived from the formula L/D/Wt. where L is load in pounds, D is deflection in inches, and Wt. is expressed in pounds.

(In the table both of the (A) samples were cut from one panel while the (B) samples were taken from another panel.)

It will be seen from the test data that the rib portions greatly increase the stiffness of the novel hardboard. Other tests have proven that the increase in stilfness (resistance to deflection) is due to the molded composition of the ribs. For example, a specimen sawn so that the tube. casing is ruptured lengthwise breaks under a load of about '50 pounds while a similar sample containing an unruptured rib will require a load of about 81 pounds before breaking. In the above-described hardboard sheets,

the molded rib portions are spaced apart about three inches. Additional tests have shown that the ribs may be spaced closer together or farther apart to obtainproducts having greater or less st-ifiness, as desired, In fact, knowing the relative. stiffness of a comparable flat sheet as well as the stiffness of a rib, portion, the ultimate stiltness of any of the. novel rib-containing sheefarticles may be predicted with surprising accuracy and the ribs'spaced accordingly. Of course, where the strength of the product is of no particular consequence, the rib spacing may be'predicated solely on thedesired decorative efiect.

The dimensional stability of the novel products of the inventionwas tested-by measuring the amount of buckle orwarp of oil impregnated samples subjected to an atmosphere of 90 F. and 90% relative humidity for varying periods of time. All samples were eight inches wide, inch thick, and were restrained on 17.0 inch centers The rib-containing samples each contained two"- rlbs'. Table 2 presents the test data.

4 Table 2 Buckle, Inches Samples After 48 Hours After 72 Hours After 96 Hours Flat Panel:

Panel with Ribs:

It will be seen, therefore, that the conventional flat hardboard sheet warped more than twice as much as the sheet containing ribs. It may be stated that the novel hardboar-d of the present invention, characterized by its rib portions which resemble batten strips, represents a more dimensionally stable hardboard sheet than those known at the present time. While it is not known with certainty, it is believed that the stresses which arise in the dense portions of the novel sheets during use are largely absorbed by the areas of low density. Thus the sheets exhibit markedly improved dimensional stability over prior art products. As further demonstration of this, samples of both rib-containing sheets and conventional fiat hardboard were mounted on a test fence and exposed to the elements for several months. Visual inspection has indicated that the rib-containing specimens have not warped appreciably. To the contrary, however,

the conventional samples warped and buckled so badly as to fail the test.

It Will be' noted in the modification of the invention shown in FIG. 4 that the edges 18 of the rib portion are rounded. It has been found that a minimum radius of curvature of about inch insures the most complete consolidation of the rib although, obviously, larger radii may be resorted to. Thepoint of juncture 20 of the rib portion and flat sheet portion is preferably rounded on a minimum radius of inch. This curvature substantially eliminates fiber rupture during the sheet pressing treatment and adds attractiveness to the finished sheet appearance. However, as illustrated in FIG. 3 juncture points 22 may, where desired, be of a more angular configuration.

Thus, it will be clear to those skilled in the art that the present invention provides a method of preparing a decorative lignocellulose hardboard characterized byoutstanding dimensional stability and greatly improved stiffness and load bearing ability. The novel product, containing integral rib portions which present the appearance of batten strips, represents a decided advance in the manufacture of hardboard.

We claim:

1. A lignocellulose hardboard sheet article characterized by improved stifiness. and dimensional stability which comprises an integral, consolidated, dense fibrous sheet having one flat surface, the other sheet surface containing a plurality of spaced-apart parallel rib portions interconnected by depressed flat sheet portions, each of said rib portions comprising essentially an enlarged molded area having a relatively dense hard shellenclosing a low specific gravity core, and said depressed flat portions being of substantially uniform specific gravity.

2. A lignocellulose hardboard sheet article characterizedby improved stillness and dimensional stability which comprises an integral, consolidated, dense fibrous sheet 7 having one flat surface, the other sheet surface containing 3. A lignocellulose hard-board sheet article, charactcrized by improved stiffness and dimensional stability which comprises an integral, consolidated, dense fibrous sheet having one fiat surface, the other sheet, surface containing a plurality of spaced-apart parallel rib portions interconnected by depressed flat sheet portions, said rib portions comprising a relatively dense shell having a specific gravity of about 0.77-0.93 enclosing a low density core having a specific gravity of about 0.60-0.72, and said depressed fiat portions having a uniform specific gravity of about 1.02-1.15.

4. A lignocellulose hardboard sheet article characterized by improved stiffness and dimensional stability which comprises an integral, consolidated, dense fibrous sheet containing polymerized drying oil and having one fiat surface, the other sheet surface containing a plurality of spaced-apart parallel rib portions interconnected by depressed fiat sheet portions, each of said rib portions comprising essentially an enlarged molded area having a relatively dense hard shell enclosing a low density core, and said depressed fiat portions being of substantially uniform density, said sheet article characterized by stiffness improvement of about 60% to about 100% as compared with a similar flat sheet article.

5. A lignocellulose hardboard sheet article as in claim 4, and wherein the juncture of said rib portions and said flat sheet portions occurs as a curve having a minimum radius of about inch.

6. A lignocellulose hardboard sheet article characterized by improved stiffness and dimensional stability which comprises an integral, consolidated, dense fibrous sheet having one flat surface, the other sheet surface containing a plurality of spaced-apart parallel rib portions connected by depressed flat sheet portions, said sheet containing a relatively small amount of a polymerized drying oil composition, each of said rib portions comprising essentially an enlarged molded area having a relatively dense hard shell having a specific gravity of about .77.93 enclosing a low density core having a specific gravity of about .60.72, and said depressed flat portions being of substantially uniform specific gravity of about 1.021.15, said sheet article characterized by stiffness improvement of about 60% to about 100% as compared with a similar fiat sheet article.

7. A lignocellulose hardboard sheet article characterized by improved stiffness and dimensional stability which comprises an integral, consolidated, dense fibrous sheet having one flat surface, the other sheet surface containing a plurality of spaced-apart parallel rib portions interconnected by depressed fiat sheet portions, said sheet containing a relatively small amount of a polymerized drying oil composition, each of said rib portions comprising essentially an enlarged molded area having a relatively dense hard shell enclosing a low density core, and said depressed flat portions being of substantially uniform density, said rib portions containing a substantially greater proportion of polymerized drying oil than said depressed fiat portions, and said sheet article characterized by stifiness improvement of about to about as compared with a similar flat sheet article.

8. Method of producing a dimensionally stable lignocellulose hardboard characterized by improved stiffness which comprises charging to a hydraulic press opening a felted fibrous lignocellulose sheet, consolidating said sheet under heat and pressure while one surface of said sheet is in contact with an imperforate pressing late having in its surface a plurality of spaced-apart parallel depressions whereby there is imparted to said sheet a plurality of spaced-apart parallel rib portions interconnected by depressed flat sheet portions, impregnating said consolidated sheet with a drying oil, and then baking said impregnated sheet at an elevated temperature.

9. Method of producing a dimensionally stable lignocellulose hardboard characterized by improved stiffness which comprises charging to a hydraulic press opening a wet felted fibrous lignocellulose sheet, consolidating said sheet under heat and pressure while one surface of said sheet is in contact with an imperl'orate pressing plate having in its surface a plurality of spaced-apart parallel depressions whereby there is imparted to said sheet a plurality of spaced-apart parallel rib portions interconnected by depressed flat sheet portions, impregnating said consolidated sheet with a drying oil, and then baking said impregnated sheet at an elevated temperature.

10. Method of producing a dimensionally stable lignocellulose hardboard characterized by improved stiffness which comprises charging to a hydraulic press opening a substantially dry felted fibrous lignocellulose sheet, consolidating said sheet under heat and pres-sure while one surface of said sheet is in contact with a flat imperforate pressing plate and the other surface of said sheet is in contact with an imperforate pressing plate having in its surface a plurality of spaced-apart parallel depressions whereby there is imparted to said sheet a plurality of spaced-apart parallel rib portions interconnected by depressed flat sheet portions, impregnating said consolidated sheet with a drying oil, and then baking said impregnated sheet at an elevated temperature.

References Cited in the file of this patent UNITED STATES PATENTS 1,461,337 Weiss July 10, 1923 2,120,137 Mason June 7, 1938 2,216,803 Benda Oct. 8, 1940 2,248,233 Heritage July 8, 1941 2,314,162 Reinhardt Mar. 16, 1943 2,766,162 Boehm et al. Oct. 9, 1956 

1. A LIGNOCELLULOSE HARDBOARD SHEET ARTICLE CHARACTERIZED BY IMPROVED STIFFNESS AND DIMENSIONAL STABILITY WHICH COMPRISES AN INTEGRAL, CONSOLIDATED, DENSES FIBROUS SHEET HAVING ONE FLAT SURFACE, THE OTHER SHEET SURFACE CONTAINING A PLURALITY OF SPACED-APART PARALLEL RIB PORTIONS INTERCONNECTED BY DEPRESSED FLAT SHEET PORTIONS, EACH OF SAID RIB PORTIONS COMPRISING ESSENTIALLY AN ENLARGED MOLDED AREA HAVING A RELATIVELY DENSE HARD SHELL ENCLOSING A LOW SPECIFICE GRAVITY CORE, AND SAID DEPRESSED FLAT PORTIONS BEING OF SUBSTANTIALLY UNIFORM SPECIFIC GRAVITY. 